Knowledge and understanding of the uses, strengths,
limitations, and other characteristics of structural
metals is vital to properly construct and maintain any
equipment, especially airframes. In aircraft maintenance
and repair, even a slight deviation from design
specification, or the substitution of inferior materials,
may result in the loss of both lives and equipment. The
use of unsuitable materials can readily erase the finest
craftsmanship. The selection of the correct material for
a specific repair job demands familiarity with the most
common physical properties of various metals.

Properties of Metals

Of primary concern in aircraft maintenance are such
general properties of metals and their alloys as hardness,
malleability, ductility, elasticity, toughness,
density, brittleness, fusibility, conductivity contraction
and expansion, and so forth. These terms are explained
to establish a basis for further discussion of structural
metals.

Hardness

Hardness refers to the ability of a material to resist
abrasion, penetration, cutting action, or permanent
distortion. Hardness may be increased by cold working
the metal and, in the case of steel and certain aluminum
alloys, by heat treatment. Structural parts are often
formed from metals in their soft state and are then heat
treated to harden them so that the finished shape will be
retained. Hardness and strength are closely associated
properties of metals.

Strength

One of the most important properties of a material is
strength. Strength is the ability of a material to resist
deformation. Strength is also the ability of a material
to resist stress without breaking. The type of load or
stress on the material affects the strength it exhibits.

Density

Density is the weight of a unit volume of a material.
In aircraft work, the specified weight of a material per
cubic inch is preferred since this figure can be used in
determining the weight of a part before actual manufacture.
Density is an important consideration when
choosing a material to be used in the design of a part
in order to maintain the proper weight and balance of
the aircraft.

Malleability

A metal which can be hammered, rolled, or pressed
into various shapes without cracking, breaking, or
leaving some other detrimental effect, is said to be
malleable. This property is necessary in sheet metal
that is worked into curved shapes, such as cowlings,
fairings, or wingtips. Copper is an example of a malleable
metal.

Ductility

Ductility is the property of a metal which permits it to
be permanently drawn, bent, or twisted into various
shapes without breaking. This property is essential for
metals used in making wire and tubing. Ductile metals
are greatly preferred for aircraft use because of their
ease of forming and resistance to failure under shock
loads. For this reason, aluminum alloys are used for
cowl rings, fuselage and wing skin, and formed or
extruded parts, such as ribs, spars, and bulkheads.
Chrome molybdenum steel is also easily formed into
desired shapes. Ductility is similar to malleability.

Elasticity

Elasticity is that property that enables a metal to return
to its original size and shape when the force which
causes the change of shape is removed. This property
is extremely valuable because it would be highly
undesirable to have a part permanently distorted after
an applied load was removed. Each metal has a point
known as the elastic limit, beyond which it cannot be
loaded without causing permanent distortion. In aircraft
construction, members and parts are so designed that the maximum loads to which they are subjected will
not stress them beyond their elastic limits. This desirable
property is present in spring steel.